Influence of Ligand Flexibility on the Electronic Structure of Oxidized

Article pubs.acs.org/IC

Influence of Ligand Flexibility on the Electronic Structure of Oxidized NiIII-Phenoxide Complexes Minoru Kawai,†,‡ Takahide Yamaguchi,‡ Shigeyuki Masaoka,§ Fumito Tani,∥ Takamitsu Kohzuma,‡ Linus Chiang,⊥ Tim Storr,⊥ Kaoru Mieda,# Takashi Ogura,# Robert K. Szilagyi,&,∇ and Yuichi Shimazaki*,† †

College of Science and ‡Graduate School of Science and Engineering, Ibaraki University, Bunkyo, Mito 310-8512, Japan § Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan ∥ Institute for Materials Chemistry and Engineering, Kyushu University, Higashi-ku, Fukuoka 812-8581, Japan ⊥ Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada # Picobiology Institute, Graduate School of Life Science, University of Hyogo, Sayo, Hyogo 679-5184, Japan & Department of Chemistry, Montana State University, Bozeman, Montana 59717, United States S Supporting Information *

ABSTRACT: One-electron-oxidized NiIII-phenoxide complexes with salen-type ligands, [Ni(salen)py2]2+ ([1en-py]2+) and [Ni(1,2-salcn)py2]2+ ([1cn-py]2+), with a five-membered chelate dinitrogen backbone and [Ni(salpn)py2]2+ ([2pn-py]2+), with a six-membered chelate backbone, have been characterized with a combination of experimental and theoretical methods. The fivemembered chelate complexes [1en-py]2+ and [1cn-py]2+ were assigned as NiIII-phenoxyl radical species, while the six-membered chelate complex [2pn-py]2+ was concluded to be a NiIIbis(phenoxyl radical) species with metal-centered reduction in the course of the one-electron oxidation of the NiIII-phenoxide complex [2pn-py]+. Thus, the oxidation state of the one-electronoxidized NiIII salen-type complexes depends on the chelate ring size of the dinitrogen backbone. phenolate moiety, affording a NiII-phenoxyl radical species, whereas addition of exogenous ligands affords five- or sixcoordinate complexes leading to a NiIII-phenoxide species.13−21 Many other redox-active group 10 complexes, especially those containing Pd or Pt complexes, have been reported as catalysts for conversion of organic substrates22 and as antitumor-active complexes.23,24 In these complexes, the metal valence states can reach +IV.25 Although some Ni complexes of +IV formal oxidation state have been previously reported as intermediates in catalytic reactions,26 few highvalent Ni(IV) species have been characterized. Further, Ni enzymes exhibit oxidation states from NiI to NiIII during catalysis, with no evidence for the +IV oxidation state.24,27 Therefore, characterization of the electronic structure of highvalent Ni complexes could lead to the development of new catalytic reactivity. Interestingly, two-electron-oxidized NiII complexes show different valence states, reflecting the “formal” and “experimental” oxidation states.28,29 For example, some two-electron-

1. INTRODUCTION The oxidation chemistry of metal complexes with redox-active ligands has been widely developed in recent years, affording insights into reaction mechanisms.1 In particular, the tyrosyl radical has attracted much attention, due to its role as an organic redox-active cofactor in galactose oxidase (GO).2,3 GO is a mononuclear copper oxidase that catalyzes the two-electron oxidation of primary alcohols to aldehydes, and the active form of GO exists as a CuII-phenoxyl radical.4,5 In general, an “experimental” valence state of a metal complex could be different from the “formal” oxidation state, especially in species containing redox-active ligands.1,3,4,6,7 Depending on the relative energies of the redox-active orbitals, metal complexes with noninnocent ligands can be assigned to two limiting descriptions, either metal−ligand radical (Mn+(L•)) or highvalent metal−anionic ligand (M(n+1)+(L−)) complexes.7−12 One-electron-oxidized nickel(II) salen-type complexes are known to exist in either form, depending on the salen ligand composition and availability of additional coordinating ligands.13−18 The coordination geometry of the nickel ion determines that the oxidation locus on square-planar fourcoordinate NiII-phenoxide complexes is dominantly the © 2014 American Chemical Society

Received: May 20, 2014 Published: September 25, 2014 10195

dx.doi.org/10.1021/ic501181k | Inorg. Chem. 2014, 53, 10195−10202

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oxidized NiII complexes containing phenoxide ligands can be assigned to a NiII-bis(phenoxyl radical) species.6,29 Recently, a one-electron-oxidized NiIII complex containing a Schiff base ligand with two phenoxide moieties was reported, but the “experimental” valence state of the oxidized complex could not be determined.20 Obtaining nickel complexes of higher valence states is still a challenge in coordination chemistry. Herein we present the valence state of a series of oneelectron oxidized nickel(III) phenoxide complexes with fiveand six-membered dinitrogen chelate backbones: [Ni(salen)py2]2+ ([1en-py]2+; Scheme 1), [Ni(1,2-salcn)py2]2+ ([1cn-

HOKUTO DENKO HZ-5000 automatic polarization system. A glassycarbon electrode and a platinum wire were used as working and counter electrodes, respectively, with an Ag/Ag+ reference electrode used at −50 °C in all the experiments. The correction of the reference electrode was evaluated by standard procedures and referenced against the ferrocene/ferrocenium redox couple. Resonance Raman spectra were obtained with a polychromator (MC-100DG, Ritsu Oyo Kogaku), a Kr+ laser (BeamLok 2060, Spectra-Physics) for 413.1 nm excitation, and a CCD detector (Symphony, CCD-1024x256-OPEN1LS, HORIBA Jobin Yvon) cooled with liquid N2. A holographic super notch filter (Kaiser Optical Systems) was used to minimize the Rayleigh scattering. Spectra of solvated samples were collected in spinning cells (0.5 cm diameter, 330 rpm) at −60 °C. The laser power was 20 mW. A 135° backscattering geometry was adopted, and the data accumulation time was 30 min. Peak frequencies were calibrated relative to indene standards (accurate to ±1 cm−1). 2.3. Typical Experiment for Electrochemical Oxidation of NiIII-Phenoxide Complexes. [1cn]SbF6 (8.4 mg, 0.01 mmol) and tetra-n-butylammonium perchlorate (428 mg, 1.25 mmol) were dissolved in 50 mL of CH2Cl2 (0.2 mM). A 40 μL portion of pyridine (0.50 mmol) was added to the solution, and the resulting solution was oxidized by 1.2 V electrolysis at −78 °C. The oxidation process was checked by coulometry. When more than 0.95 electron was transferred from the complexes, the electrolysis was stopped and the resulting solution was identified by UV−vis−NIR absorption and resonance Raman spectroscopy as one-electron-oxidized NiIII-phenoxide complexes. 2.4. Typical Experiment for Chemical Oxidation of NiIIIPhenoxide Complexes with [Ru(bpy)3](PF6)3. [1cn]+ (3.4 mg, 0.004 mmol) was dissolved in 10 mL of CH2Cl2 (0.4 mM). To this solution was added 16 μL of pyridine (0.20 mmol), and the solution was cooled to −50 °C. The 4 mM CH3CN solution of [Ru(bpy)3](PF6)3 (1 mL, 0.004 mmol) was added to the solution at −50 °C. 2.5. XAS Experiments. The Ni K-edge XAS spectra were collected at the Stanford Synchrotron Radiation Laboratory under the ring condition of 3.0 GeV and ∼500 mA in top-off mode. The measurement utilized the 20-pole wiggler beamline 7-3 with unfocused beam and a Si(220) monochromator with ϕ = 90° alignment. The higher harmonic components from the monochromator were eliminated by a vertically collimating premonochromator harmonic rejection mirror. Solution samples were prepared in pinhole Delrin sample cells with 1 mil Kapton tape. The loaded cells were frozen by liquid nitrogen after a few seconds of mixing upon oxidation. The temperature of solution samples was maintained at 10 K during measurements using a dedicated Oxford liquid He flow cryostat. Incident (I0) and transmission (I1 and I2) beam intensities were measured using N2-filled ion chambers at 1100 V potential. The data were collected in fluorescence mode using a 30-element Ge solid state detector (Canberra) with a Soller slit frame equipped with a 3 μm Co Z-1 filter. The Ni calibration foil was placed between the second (I1) and third (I2) ion chambers. The energy was calibrated by setting the first inflection point along the rising edge of the Ni-foil spectra to 8333.0 eV.30 At least three spectra were averaged for XANES and EXAFS analyses. Only a negligible effect of radiation damage was observed during data normalization. The energy calibration and averaging of spectra were performed using SIXPACK.31 The reduced data were processed for background subtraction, data renormalization, extraction of EXAFS oscillation, and collection of Fourier transform data using Athena.32 For EXAFS analysis, the theoretical EXAFS scattering pathways were calculated by FEFF version 6.033 and used in Artemis.32 The structural parameters were refined by allowing the pathway length (R) and the Debye−Waller factor to vary for each scattering path. Given the small difference in atomic number, we could not differentiate between Ni−O and Ni−N scattering pathways. The degeneracy of Ni−O/N scattering for [1cn]+ and [2pn]+ was set to 4, due to 4 comparable distances for Ni−O and Ni−N with salen-type ligands. The variations of the Ni−O and Ni−N bond lengths in complexes [1cn]+ and [2pn]+ are within 0.03 Å (Table 4). The EXAFS features of complexes [1cn-py]+ and [2pn-py]+ were fitted with an additional Ni−N distance with double degeneracy due to the presence

Scheme 1. Structures of the salen-Type Five-Membered Dinitrogen Chelate Complexes Ni(salen) (1en) and Ni(1,2salcn) (1cn) and Six-Membered Dinitrogen Chelate Complexes Ni(salpn) (2pn) and One-Electron Oxidation of Their NiIII Complexes

py]2+), and [Ni(salpn)py2]2+ ([2pn-py]2+). The six-membered chelate NiIII-phenoxide complex [2pn-py]+ is more distorted than both [1en-py]+ and [1cn-py]+, while their physicochemical properties are similar.15,17 One-electron oxidation of these NiIIIphenoxide precursors has been concluded to afford complexes with different electronic structures depending on the dinitrogen chelate ring size; [1en-py]2+ and [1cn-py]2+ are the first examples of NiIII-phenoxyl radical complexes, while [2pn-py]2+ is a NiII-bis(phenoxyl radical) species formed with concomitant reduction of the NiIII center.

2. EXPERIMENTAL SECTION 2.1. Syntheses of Ni Complexes. Synthesis of the NiII complexes Ni(1,2-salcn) (1cn) and Ni(salpn) (2pn) and their one-electronoxidized complexes [Ni(salen)]SbF6 ([1 en] +), [Ni(salcn)]SbF6 ([1 cn ] + ), and [Ni(salpn)]SbF 6 ([2 pn] + ) have been reported previously.16,17 The NiIII complexes [Ni(salen)py2]SbF6 ([1en-py]+), [Ni(1,2-salcn)py2]SbF6 ([1cn-py]+), and [Ni(salpn)]SbF6 ([2pn-py]+) were prepared by addition of 50 equiv of pyridine to CH2Cl2 solutions of [1]+ and [2]+.16,17 2.2. General Methods. UV−vis−NIR spectra were obtained with a JASCO V-670 spectrophotometer, equipped with a liquid nitrogen cooled cryostat (UNISOKU CoolSpeK UV USP-203-A), from −80 to +30 °C. EPR spectra were collected by using a Bruker EMXplus spectrometer operating with a premium X-band (9.5 GHz) microwave bridge. Low-temperature measurements of frozen solutions were carried out by using a Bruker helium temperature-control system and a continuous-flow cryostat. Samples for X-band measurements were placed in outer-diameter sample tubes (4 mm) with sample volumes of 300 μL. Electrolysis and cyclic voltammetry (CV) was performed on a 10196


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of the axially coordinated N of pyridine. The EXAFS of [2pn-py]2+ were analyzed with fitting parameters similar to those used for [1cnpy]2+ without being able to obtain a reasonable fit. The degeneracy of Ni−N/O−C multiple scattering pathways were also increased to accompany the C atom bonding to N of pyridine (Table 3). The numbers of independent points for EXAFS analyses were calculated following Nind = (2(Δk)(ΔR))/π, where ΔR = 2 and Δk was more than 7.65, and thus Nind was more than 9.7. The number of variables was maintained to be less than Nind. 2.6. Calculation Details. Geometry optimizations were performed using the Gaussian 09 program (Revision D.01),34 the B3LYP functional,35,36 the 6-31g(d) basis set, and a polarized continuum model (PCM) for CH2Cl2 (dielectric ε = 8.94).37 This functional/ basis set combination has been shown previously to give the best agreement with solid-state structural data for this class of compounds.10,16,38 Frequency calculations at the same level of theory confirmed that the optimized structures were located at a minimum on the potential energy surface. Single-point calculations for energetic analysis were performed using the BLYP39 functional, the 6-311g(d) basis set, and a polarized continuum model (PCM) for CH2Cl2 (dielectric ε = 8.94).36 For the one-electron-oxidized Ni(III) complexes the fragment guess function in Gaussian was used to search for plausible electronic structures for the complexes. The resulting electronic solutions are detailed in the Supporting Information (Tables S2−S4), and in many cases different initial guesses converged to a common electronic structure.

Figure 1. Cyclic voltammograms for NiIII complexes (A) [1cn-py]+, (B) [1en-py]+, and (C) [2pn-py]+. Conditions: in CH2Cl2, 223 K, 0.1 M [NBu4]ClO4 as the supporting electrolyte, a carbon electrode as the working electrode, a Pt electrode as the counter electrode, a Ag/Ag+ electrode as the reference electrode, scan rate 100 mV/s.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of Precursor NiIII-Phenoxide Complexes. Previously we and others reported that addition of exogenous ligands, such as pyridine, to a solution of NiII-phenoxyl radical complexes of salen-type ligands affords NiIII-phenoxide complexes with a six-coordinate geometry, and the association constants (log β) were estimated to be 7.3 ± 0.2, 6.8 ± 0.2, and 7.3 ± 0.2 M−2 at 223 K for [1cn]+, [1en]+, and [2pn]+, respectively (Figure S1, Supporting Information). Further addition of pyridine (up to 50 equiv) to solutions of the NiII-phenoxyl radical complexes did not result in further spectral changes; therefore, the NiIII-phenoxide complexes were prepared by addition of 50 equiv of pyridine to the NiII-phenoxyl radical species in CH2Cl2 below 223 K.16,17 Cyclic voltammograms of all NiIII complexes in CH2Cl2 exhibited one reversible redox wave (0.85 V for [1cn-py]+, 0.82 V for [1en-py]+, and 0.46 V for [2pn-py]+ versus Fc/Fc+) in the higher range from the rest potential (0.39, 0.46, and 0.37 V for [1cn-py]+ [1en-py]+, and [2pn-py]+, respectively) (Figure 1, Table 1). The CVs of the NiIII complexes were different from those of the precursor NiII complexes 1 and 215,17 due to differences in the oxidation state, coordination number, and geometry. The redox potential of [2pn-py]+ is similar to the first ligand-based redox potential of complexes 1 and 2, suggesting that [2pn-py]+ may show a similar ligand-centered oxidation process in comparison to complexes 1 and 2. On the other hand, the oxidation potential of the five-membered chelate complexes [1-py]+ (both cn and en) are ca. 0.3 V higher than that of [2pn-py]+, suggesting that the oxidation locus of [1-py]+ may be different from that of [2pn-py]+. 3.2. Formation and Properties of One-ElectronOxidized NiIII Species. Electrochemical oxidation of the five-membered chelate complexes [1cn-py]+ and [1en-py]+ at 1.2 V and [2pn-py]+ at 1.0 V in CH2Cl2 solution at −80 °C revealed a transfer of more than 0.9 electron per molecule with a gradual color change to dark red and purple for [1-py]2+ and [2-py]2+, respectively. The UV−vis−NIR spectra of the oneelectron-oxidized species exhibited characteristic new peaks

Table 1. CV Data for Ni-salen Type Complexes complex

[1cn]

[1cn-py]+a

[1en]

[1en-py]+a

[2pn]

[2pn-py]+a

E1/2 (V) ref

0.69 14

0.85

1.05 40

0.82

0.85 16

0.56

Conditions: 1 mM complex, 0.1 M NBu4ClO4, scan rate 100 mV s−1, CH2Cl2, 223 K. a

(Figure 2, Table 2). Chemical oxidation by [RuIII(bpy)3](PF6)341 afforded the same oxidized species as for electro-

Figure 2. UV−vis−NIR spectral changes from NiIII-phenoxide complexes to one-electron-oxidized forms using controlled electrolysis: (A) [1cn-py]+ (black line), [1cn-py]2+ (red line); (B) [1en-py]+ (black line), [1en-py]2+ (red line); (C) [2pn-py]+ (black line), [2pnpy]2+ (red line). Conditions: 223 K, 0.1 mM solution, 1 cm cell.

chemical oxidation (Figure S2, Supporting Information), as concluded from the similarity of the absorption spectra in the visible and NIR regions. In the UV−vis region, we only observed small differences in the spectra; those of the 5-chelate dinitrogen complexes [1cn-py]2+ and [1en-py]2+ were essentially the same. On the other hand, a notable difference is observed in the NIR region, where a weak band was observed at ca. 6000 cm−1 (ε = 500−800 M−1cm−1) for the five-membered chelate complexes [1-py]2+ but [2pn-py]2+ showed no significant NIR bands. The variation in the NIR spectral features suggests a difference in the detailed electronic structures of the oxidized complexes, such as a difference in oxidation locus, metal versus ligand, and localization and delocalization of the oxidation 10197


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Table 2. UV−Vis−NIR Data of [1cn-py]n+, [1en-py]n+, and [2pn-py]n+ (n = 1, 2) complex cn

[1 py]

+

[1cn-py]2+ [1en-py]+ [1en-py]2+ [2pn-py]+ [2pn-py]2+

ν/cm−1 (ε/M−1 cm−1) 29300 (12800), 22900 (6900), 19900 (8700), 12600 (400), 7600 (1100) 28200 (9100), 25000 (8000), 23500 (10100), 19100 (5100), 15900 (2600), 12200 (1800), 5900 (800) 23100 (9000), 20200 (8700), 7700 (1200) 26800 (6800), 24800 (8200), 23500 (10100), 19500 (4200), 16000 (2500), 12200 (1800), 6000 (500) 26900 (8500), 25600 (7300), 23800 (5200), 18500 (6700), 11400 (200), 6000 (2200) 26700 (7100), 24800 (8900), 23400 (11200), 17600 (2800), 11200 (1400)

locus. The UV−vis−NIR and CV results suggest that the electronic structures of the one-electron-oxidized NiIII complexes differ depending on the chelate ring size. The oxidized species could be reduced to the original NiIIIphenoxide species by addition of 1 equiv of ferrocene. This result indicated the reversibility of the redox process, which is in good agreement with the observation of a reversible redox wave by CV (Figure S3, Supporting Information). All three oxidized NiIII complexes were X-band EPR silent at 4 K in the range 0−500 mT (Figure S4, Supporting Information), suggesting an electronic structure different from that of the NiIII species, which has characteristic EPR signals of axially symmetrical S = 1/2 at gav = ca. 2.15.42 These EPR data suggest either that there is antiferromagnetic coupling of the radical spins in oxidized NiIII-radical species or that the triplet energy difference is outside of the range of X-band EPR. An S = 0 ground state has been measured for a large number of Ni bisligand radical systems, with the strength of antiferromagnetic coupling dependent on the degree of exchange through the central metal ion.6,28,43 3.3. XAS Analyses. In order to experimentally evaluate the changes in the nickel oxidation states and the nickel coordination geometry of five- and six-membered chelate ring complexes, we carried out nickel K-edge X-ray absorption spectroscopic (XAS) measurements for frozen solutions at 10 K. The Ni pre-edge (1s to 3d transition) and rising edge (1s to 4p transition) for the NiII phenoxyl radical complexes [1cn]+ and [2pn]+ and their corresponding NiIII pyridine adducts [1cnpy]+ and [2pn-py]+ (Figure 3, Table 3) showed distinct differences, which are attributable to a change in the metal oxidation state from NiII to NiIII upon oxidation.44 The disappearance of a well-defined shoulder along the rising edge further suggests the increase in the coordination number from 4 to 6.44,45 The change in the pre-edge feature position (from 8333.8 eV for [1cn]+ to 8335.1 eV for [1cn-py]+ and from 8333.4 eV for [2pn]+ to 8334.2 eV for [2pn-py]+) (Table 3) is due to the combined effect of increasing the Ni effective nuclear charge by a metal-centered oxidation and the change from square-planar to octahedral coordination.44 The rising-edge inflection points as a measurement of Ni effective nuclear charge at 8346.5 and 8346.6 eV for [1cn-py]+ and [1cn-py]2+, respectively, are practically identical. Similarly, the pre-edge features for [1cn-py]+ and [1cn-py]2+ (8334.9 eV) showed no significant shift. These results indicate that the oxidation locus is not metal-based for this derivative. On the other hand, [2pnpy]2+ exhibited counterintuitive changes in rising-edge and preedge energy positions relative to [2pn-py]+, with a shift to lower energy by 0.7 and 0.8 eV, respectively (pre-edge, 8333.4 eV in

Figure 3. XANES of four-coordinate NiII-phenoxyl radical and sixcoordinate NiIII-phenolate and their oxidized complexes and expansion of the pre-edge region (inset): (A) [1cn]+ (blue line), [1cn-py]+ (black line), [1cn-py]2+ (red line); (B) [2pn]+ (blue line), [2pn-py]+ (black line), [2pn-py]2+ (red line).

Table 3. XANES and EXAFS Data of Ni Complexes pre-edge energy (eV) (intensity)

rising-edge energy (eV)

[1 ] [1cn]+ [1cn-py]+

8334.1 (0.054) 8333.8 (0.048) 8335.1 (0.036)

8338.9 8339.1 8346.5

[1cn-py]2+

8334.9 (0.037)

8346.6

[2pn] [2pn]+ [2pn-py]+

8333.7 (0.053) 8333.4 (0.055) 8334.2 (0.037)

8338.8 8339.2 8346.1

[2pn-py]2+

8333.4 (0.036)

8345.4

cn

ligation 4 4 4 2 4 2 4 4 4 2 6

N/O N/O N/O N N/O N N/O N/O N/O N N/O

R (Å)

σ (Å2)

1.84 1.83 1.88 2.20 1.93 2.16 1.87 1.86 1.91 2.12 2.06

0.00263 0.00216 0.00366 0.00176 0.00617 0.00112 0.00647 0.00730 0.00590 0.00158 0.00413

[2pn-py]2+ versus 8334.2 eV [2pn-py]+; rising edge, 8345.4 eV in [2pn-py]2+ versus 8346.1 eV [2pn-py]+) (Table 3). These spectral changes suggest a decrease in the Ni effective nuclear charge upon oxidation. The ca. 0.7 eV differences are in good agreement with the XAS differences between six-coordinated [NiII(cyclam)Cl2] and [NiIII(cyclam)Cl2]+.44 Therefore, the oxidation locus on [2pn-py]2+ is the ligand, which concomitantly reduces the NiIII center to form a NiII-bis(phenoxyl radical) complex. Extended X-ray absorption fine structure (EXAFS) analyses revealed different coordination geometries for [1cn-py]2+ and [2pn-py]2+. For the NiIII complexes [1cn-py]+ and [2pn-py]+ the four N/O donors of the salen ligand and the two N donors of the pyridines could be fit with two single-scattering shells at distances of 1.88 and 2.20 Å for [1cn-py]+ and 1.91 and 2.12 Å for [2pn-py]+, respectively (Figure 4, Table 4). The EXAFS structure of [1-py]2+ with four N/O donors at 1.93 Å and two 10198


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Figure 4. EXAFS data (blue line) and fits (red line) of R space and k space (inset): (A) [1cn-py]2+; (B) pn-py]2+.

N donors at 2.16 Å was very similar to that of [1-py]+. The slight elongation of the distances for the 4-donors in the equatorial plane parallels the observed elongation of the metal− phenoxyl radical bond relative to the metal−phenolate bond and an overall expansion of the salen coordination sphere, as shown for the localized phenoxyl radical species.46,47 On the other hand, [2pn-py]2+ showed a local coordination geometry different from that of [2pn-py]+, with all six Ni−N/O distances being best modeled at 2.06 Å.48 This structural feature closely matches the general M−L bond lengths of high-spin octahedral NiII rather than NiIII species.49 From these results, we assigned [1cn-py]2+ as a NiIII-monophenoxyl radical complex and [2pnpy]2+ as a high-spin NiII-bis(phenoxyl radical) complex. 3.4. Resonance Raman Analysis. Resonance Raman spectra of the oxidized complexes [1en-py]2+, [1cn-py]2+, and [2pn-py]2+ exhibited the characteristic phenoxyl radical ν7a band13,28,50 at 1494, 1493, and 1498 cm−1, respectively (Figure 5 and Figure S6 (Supporting Information)), suggesting that the oxidized complexes [1-py]2+ and [2pn-py]2+ could be assigned to phenoxyl radical species. Although the intensities of the ν7a bands for [1en-py]2+ and [1cn-py]2+ were very similar, the intensity of the phenoxyl radical ν7a band of [2pn-py]2+ was more intense in comparison to that of [1-py]2+. The intensity difference of the ν7a band could arise from several factors. The UV−vis absorption spectra of all the oxidized NiIII complexes around the excitation wavelength (λex 413.1 nm) exhibited very similar absorption features, but the ε value of [2pn-py]2+ is slightly larger (Table 2). The Raman band intensity of [2pnpy]2+ may be enhanced due to the more intense absorption band around the excitation wavelength. On the other hand, the number of phenoxyl radicals in [1-py]2+ and [2pn-py]2+ could be also considered; the five-membered chelate complexes [1py]2+ contain one phenoxyl radical moiety, while the sixmembered chelate complex [2pn-py]2+ can be described as a bis(phenoxyl radical).29 In addition, the phenolate ν11a band at 1529 and 1527 cm−1 remained in both [1en-py] 2+ and [1cnpy]2+, but the band could not be detected in [2pn-py]2+. The bands observed for [1-py]2+ suggested that [1-py]2+ can be assigned to a localized phenoxyl radical species, described as

Figure 5. Resonance Raman spectral changes from NiIII complexes to one-electron-oxidized species (λex 413.1 nm, CH2Cl2 at −60 °C): (A) [1en-py]+ (black line), [1en-py]2+ (red line); (B) [2pn-py]+ (black line), [2pn-py]2+ (red line).

NiIII-(phenolate)(phenoxyl radical). A similar observation was also reported for a one-electron-oxidized CuII salen-type complex, which exhibits a localized phenoxyl radical.12 On the other hand, disappearance of the phenoxide band in [2pnpy]2+ suggests that two phenoxide moieties were oxidized to a bis(phenoxyl radical) species. The bis(phenoxyl radical) state in [2pn-py]2+ led to the intense ν7a phenoxyl radical Raman band of the complex in comparison to that of [1-py]2+. From these results, the complex [2pn-py]2+ was assigned as NiII-bis(phenoxyl radical), which is in line with the assignment from the XAS analyses. 3.5. Theoretical Calculations. Density functional theory (DFT) calculations on the complexes [1cn-py]+, [1en-py]+, and [2pn-py]+ and their oxidized analogues were undertaken to better understand the electronic structures. The predicted geometries and electronic structures for the complexes [1cnpy]+ and [1en-py]+, containing five-membered chelate backbones, and their oxidized forms were found to be essentially identical (Figures S7−S13 and Tables S1−S3, Supporting Information). We have thus focused the discussion on the cyclohexyl backbone analogue and comparison to the propyl backbone derivative, in order to compare the predicted metrical parameters with the EXAFS fitting. The predicted metrical parameters for [1cn-py]2+ and [2pn-py]2+ (starting from the optimized [1cn-py]+ and [2pn-py]+ geometries, respectively) reproduced well the coordination sphere bond lengths, including the structural changes upon oxidation to the dications [1cn-py]2+ and [2pn-py]2+ (Table 4 and Figures 6 and 7). At the B3LYP36/6-31g(d) level of theory for all atoms, the Ni coordination distances predicted by geometry optimization of [1cn-py]+ and [1cn-py]2+ and of [2pn-py]+ and [2pn-py]2+ matched those of the EXAFS analyses within 0.024 Å for distances of the 4N/O nickel donors. Energetic analysis at the

Table 4. Comparison of the EXAFS Fitting Data with DFT Calculations EXAFS Ni−N/O (Å) [1cn]+ [1cn-py]+ [1cn-py]2+ [1en-py]+ [1en-py]2+ [2pn]+ [2pn-py]+ [2pn-py]2+

DFT Ni−Npy (Å)

1.83 1.88 1.93

2.20 2.16

1.86 1.91 2.06

2.12 2.06 10199

Ni−O (Å)

Ni−Nsal (Å)

1.844 1.886/1.887 1.963/1.856 1.885/1.886 1.960/1.853 1.851 1.897 2.064

1.841 1.899/1.902 1.910/1.907 1.898/1.891 1.904/1.900 1.870 1.935 2.043

Ni−Npy (Å) 2.266/2.268 2.171/2.301 2.277/2.240 2.173/2.280 2.270 2.180


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complexes due to the ligand flexibility. The complexes with a five-membered chelate ring [1-py]2+ could be assigned to a NiIII-phenoxyl radical species, while the six-membered chelate complex [2pn-py]2+ was concluded to be a high-spin d8 NiIIbis(phenoxyl radical) species. The more rigid coordination environment of the five-membered chelate ligands salen and salcn in [1-py]2+ likely provides coordinate bond lengths that stabilize the NiIII oxidation state. On the other hand, the increased flexibility of the six-membered chelate in [2pn-py]2+ stabilizes the bis-phenoxyl radical species, and the longer coordination sphere bond lengths associated with this form, in comparison to [1-py]2+. The difference of oxidation locus depending on the chelate ring size was also observed in oneelectron-oxidized CuII salen-type complexes.12 The observed metal-centered reduction in the course of the one-electron oxidation is rare.54 As an extension of the studies on the oxidation chemistry of nickel and other transition-metal salentype complexes, further investigations of the high-valent nickel complexes are in progress in our laboratories.

Figure 6. Predicted spin density plot for the S = 1 solution for [1cnpy]2+ corresponding to a NiIII-phenoxyl radical structure. See the Experimental Section for calculation details.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis−NIR spectral change for formation of Ni(III) complexes by addition of pyridine (Figure S1), UV−vis−NIR spectra of oxidized Ni(III) complexes by Ru(III) oxidation (Figure S2), UV−vis−NIR spectral change for chemical reduction of oxidized Ni(III) species (Figure S3), EPR of oxidized Ni(III) complexes (Figure S4), EXAFS data of Ni(III) and their oxidized complexes (Figure S5), resonance Raman spectra of oxidized Ni(III) complexes (Figure S6), and DFT calculation data of Ni(III) and their oxidized complexes (Figures S7−S16, Tables S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. Predicted spin density plot for the ground state for [2pnpy]2+, showing an antiferromagnetially coupled NiII-bis(phenoxyl radical) species. See the Experimental Section for calculation details.

BLYP39/6-311g(d) level of theory revealed a number of electronic states for [1cn-py]2+ (Table S2, Supporting Information); however, the electronic ground state is predicted to be a low-spin d7 NiIII localized phenoxyl radical electronic solution (S = 1, Figure 6), which also provides the best geometric match to the EXAFS data. The assignment of [1cnpy]2+ as a NiIII localized phenoxyl radical by a combination of spectroscopic methods and theoretical calculations agrees well with the lower intensity of the NIR band of [1cn-py]2+ at 5900 cm−1 and [1en-py]2+ at 6000 cm−1 (Figure 2), in comparison to the delocalized NiII-monophenoxyl radical [1cn]+,13,51 suggesting a phenolate to phenoxyl radical transition described as class II localization by the Robin and Day classification.11,12,52,53 The DFT calculations for [2pn-py]2+ resulted in a number of electronic solutions of similar energy; however no NiIII ligand radical electronic solution was located for the propyl backbone (Table S4, Supporting Information). The calculated electronic ground state for the complex [2pn-py]2+ was an antiferromagnetically coupled NiII-bis(phenoxyl radical) species (Figure 7). An electronic solution consisting of a high-spin NiII center ferromagnetically coupled to two phenoxyl radicals (+1.4 kcal/ mol) and a high-spin NiII center exhibiting one ferromagnetic and one antiferromagnetic interaction to two phenoxyl radicals (+0.6 kcal/mol) was slightly higher in energy (Table S4). The calculated high-spin NiII-bis(phenoxyl radical) electronic structure for [2pn-py]2+ is in good agreement with the interpretation of the resonance Raman, the pre-edge, and the EXAFS structural features of [2pn-py]2+. In addition, [2pn-py]2+ exhibited no significant bands in the NIR region, which was in accord with the assigned electronic structure.28,50

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.S.: [email protected]. Present Address ∇

Department of Analytical Chemistry, University of Pannonia, Veszprem, Hungary.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research (Nos. 22550055 and 25410060 to Y.S. and 25109540 to T.O.) from the Ministry of Education, Culture, Sports, Science, Cooperative Research in Joint Studies Programs at the Institute for Molecular Science, and Technology of Japan by the Cooperative Research Program of Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University). T.S. thanks the NSERC for a Discovery Grant and Westgrid for access to computational resources. R.K.S. acknowledges his sabbatical leave from Montana State University and his current funding from the European Union and the State of Hungary, cofinanced by the European Social Fund in the framework of TÁ MOP 4.2.4.A/2-11-1-2012-0001 “National Excellence Program”. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

4. CONCLUSION We have characterized the one-electron-oxidized NiIII complexes of salen-type ligands containing five- and six-membered dinitrogen chelate rings. The difference in the ring size led to a different oxidation locus of the one-electron-oxidized NiIII 10200


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Influence of Ligand Flexibility on the Electronic Structure of Oxidized

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